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[Joeri Belis]

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CPU throttling??

[Seamus]

Just a simple question,

Would it be possible to throttle cpu when machine is in idle mode in
Linux? or is it purely a BIOS and motherboard functionality.

As you know some modern laptops in order to save power, throttle cpu
(lower the cpu clock cycles per sec) when in idle mode.

Thanks,

Seamus

Re: CPU throttling??

[Dave Jones]

On Mon, Feb 03, 2003 at 04:55:13PM +0000, Seamus wrote:

 > Would it be possible to throttle cpu when machine is in idle mode in
 > Linux? or is it purely a BIOS and motherboard functionality.
 > 
 > As you know some modern laptops in order to save power, throttle cpu
 > (lower the cpu clock cycles per sec) when in idle mode.

ACPI does this, CPUFreq does this, and if you're really lucky,
there are some APM implementations that do it.

		Dave

-- 
| Dave Jones.        http://www.codemonkey.org.uk
| SuSE Labs

Re: CPU throttling??

[Martin Hermanowski]

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On Mon, Feb 03, 2003 at 04:55:13PM +0000, Seamus wrote:
> Just a simple question,
> 
> Would it be possible to throttle cpu when machine is in idle mode in
> Linux? or is it purely a BIOS and motherboard functionality.
> 
> As you know some modern laptops in order to save power, throttle cpu
> (lower the cpu clock cycles per sec) when in idle mode.

http://www.brodo.de/cpufreq/

Works well with my P3m

Regards,
Martin

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Re: CPU throttling??

[John Bradford]

> Just a simple question,
> 
> Would it be possible to throttle cpu when machine is in idle mode in
> Linux? or is it purely a BIOS and motherboard functionality.
> 
> As you know some modern laptops in order to save power, throttle cpu
> (lower the cpu clock cycles per sec) when in idle mode.

CPU speed throttling is is something is that is currently being worked
on.

Incidently, Linux has always halted the processor, rather than spun in
an idle loop, which saves power.

John.

Re: CPU throttling??

[Valdis.Kletnieks]

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On Mon, 03 Feb 2003 17:13:02 GMT, John Bradford said:

> Incidently, Linux has always halted the processor, rather than spun in
> an idle loop, which saves power.

It's conceivable that a CPU halted at 1.2Gz takes less power than one
at 1.6Gz - anybody have any actual data on this?  Alternately phrased,
does CPU throttling save power over and above what the halt does?
-- 
				Valdis Kletnieks
				Computer Systems Senior Engineer
				Virginia Tech


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Re: CPU throttling??

[Dave Jones]

On Mon, Feb 03, 2003 at 01:57:17PM -0500, Valdis.Kletnieks@vt.edu wrote:

 > It's conceivable that a CPU halted at 1.2Gz takes less power than one
 > at 1.6Gz - anybody have any actual data on this?  Alternately phrased,
 > does CPU throttling save power over and above what the halt does?

Given that most decent implementations scale voltage as well as
frequency, yes, a lower speed will save more power.

		Dave

-- 
| Dave Jones.        http://www.codemonkey.org.uk

Re: CPU throttling??

[Martin Hermanowski]

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On Mon, Feb 03, 2003 at 01:57:17PM -0500, Valdis.Kletnieks@vt.edu wrote:
> On Mon, 03 Feb 2003 17:13:02 GMT, John Bradford said:
> 
> > Incidently, Linux has always halted the processor, rather than spun in
> > an idle loop, which saves power.
> 
> It's conceivable that a CPU halted at 1.2Gz takes less power than one
> at 1.6Gz - anybody have any actual data on this?  Alternately phrased,
> does CPU throttling save power over and above what the halt does?

If I slow down my 1GHz CPU to 732MHz, I get 15min more (195min total).
So it is not much, but noticeable.

Regards,
Martin

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Re: CPU throttling??

[Matt Reppert]

On Mon, 03 Feb 2003 13:57:17 -0500
Valdis.Kletnieks@vt.edu wrote:

> On Mon, 03 Feb 2003 17:13:02 GMT, John Bradford said:
> 
> > Incidently, Linux has always halted the processor, rather than spun in
> > an idle loop, which saves power.
> 
> It's conceivable that a CPU halted at 1.2Gz takes less power than one
> at 1.6Gz - anybody have any actual data on this?  Alternately phrased,
> does CPU throttling save power over and above what the halt does?

Yes. I have a powerpc laptop that runs at 700 MHz. If I throttle the CPU clock
speed down to 400 MHz and change nothing else the battery has noticeably longer
life; since it's running slower, it takes less power when it's active (not
halted).

Matt

Re: CPU throttling??

[John Bradford]

> > > Incidently, Linux has always halted the processor, rather than spun in
> > > an idle loop, which saves power.
> >
> > It's conceivable that a CPU halted at 1.2Gz takes less power than one
> > at 1.6Gz - anybody have any actual data on this?  Alternately phrased,
> > does CPU throttling save power over and above what the halt does?
> 
> If I slow down my 1GHz CPU to 732MHz, I get 15min more (195min total).
> So it is not much, but noticeable.

Does anybody have any data on frequency throttling on non-X86
architectures?

John.

Re: CPU throttling??

[Valdis.Kletnieks]

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On Mon, 03 Feb 2003 13:14:56 CST, Matt Reppert said:

> Yes. I have a powerpc laptop that runs at 700 MHz. If I throttle the CPU cloc
k
> speed down to 400 MHz and change nothing else the battery has noticeably long
er
> life; since it's running slower, it takes less power when it's active (not
> halted).

I knew that.  The question I asked was whether halted at 700Mhz takes more
power than halted at 400Mhz... 

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Re: CPU throttling??

[Erik Mouw]

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On Mon, Feb 03, 2003 at 07:20:34PM +0000, John Bradford wrote:
> Does anybody have any data on frequency throttling on non-X86
> architectures?

See http://www.lart.tudelft.nl/projects/scaling/ for papers describing
clock&voltage scaling on StrongARM SA-1100. The same page also has my
OLS 2002 cpufreq overview paper.


Erik

-- 
J.A.K. (Erik) Mouw
Email: J.A.K.Mouw@its.tudelft.nl  mouw@nl.linux.org
WWW: http://www-ict.its.tudelft.nl/~erik/

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Re: CPU throttling??

[Daniel Egger]

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Am Mon, 2003-02-03 um 20.24 schrieb Valdis.Kletnieks@vt.edu:

> I knew that.  The question I asked was whether halted at 700Mhz takes more
> power than halted at 400Mhz... 

PowerPC CPUs will shut down unneeded units. There's no such things as a
"hlt" instruction. The natural way to slow down the CPU causing more
sparetime to shut down single units is to throttle the dispatching of
instructions for which there's are flags in the control register set.

I've no idea where the frequency scaling should happen but it either
needs some hardware clock control or it's the mentioned dispatch
throttling. In case of the former there'd be some powersave effects
because the power drawn by a CPU is direct proportional to the
frequency. In the latter case the difference between an automatically 
sleeping 700Mhz and a throttled 700Mhz cpu at "400Mhz" should be pretty
small, in case the cpu is mostly idle.

-- 
Servus,
       Daniel

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